Print substrates are materials, such as paper substrates or polymeric substrates etc., on which images may be printed using various printing techniques such as gravure printing, lithographic printing, inkjet printing and electrostatic printing (including liquid electrostatic printing). Prior to printing an image on a print substrate, it may be desirable to provide a primer layer on the print substrate, for example to prepare the print substrate to receive an image. It may be desirable to determine the coat weight of primer on the print substrate, for example to ensure that the print substrate is suitably prepared to receive an image, while also minimising the amount of primer used. A primer may be deposited onto a print substrate in order to improve adhesion of an image to the print substrate. It may also be desirable to control the coat weight of primer on the print substrate. Some previous methods of determining the coat weight of a primer on a print substrate employ gravimetric analysis. The results obtained by gravimetric analysis may vary due to operator technique.